1. Field of the Invention
This invention relates generally to computer systems and more particularly to clock control techniques for microprocessors. The invention also relates to the measurement of propagation delays within microprocessors.
2. Description of the Relevant Art
Microprocessors or central processing units (referred to herein as "processors" or "CPU's") are typically capable of operating at much higher frequencies than the frequency at which they are actually set to operate. That is, since a processor in a computer system must allow for a worst case scenario of operating conditions, the processor is typically set to operate at a frequency lower than its actual maximum operating frequency during normal conditions. In general, unfavorable operating conditions of the processor chip may cause an increase in the time required for a particular signal to propagate through a designated portion of the microprocessor circuitry. Therefore, in order to maintain proper operation of the computer system at all times, manufacturers rate a particular processor for certain predetermined worst case conditions, and specify the maximum operating frequency based on these worst case conditions.
Thus, for example, a 33 MHz rated processor may potentially be operable at a higher frequency, such as 49.5 MHz, during favorable operating conditions. The processor, however, will not be driven at 49.5 MHz in practice because the operating conditions may not always be favorable. If the processor were driven at 49.5 MHz, failures could occur during the periods of less favorable operating conditions.
Exemplary parameters that can affect the propagation delays associated with a processor include temperature, supply voltage levels, and manufacturing variations. FIGS. 1A-1C are graphs which illustrate general relationships between these parameters and propagation delay. Referring first to FIG. 1A, as the temperature of a processor increases, the propagation delays associated with the internal circuitry of that processor also typically increase. As illustrated in FIG. 1B, supply voltage levels can also affect propagation delays associated with a processor. Generally speaking, the lower the supply voltage to the processor, the greater the propagation delays. Processing variations may also impact propagation delays, as depicted in FIG. 1C. The maximum rated frequency of a processor is typically set to ensure operability when all delay-affecting parameters reach their worst case condition concurrently.
With computing speed and performance a premium in today's market, a need exists for a processor that is configured to operate at an optimal frequency based on the current operating conditions of the processor.